Patent GB-684948-A (GB)
Title: Improvements in or relating to electrical amplifying systems
IPC: G01R13/22
Filed: 1943-09-21
Abstract: 684,948. Valve amplifying circuits. SPERRY CORPORATION. Sept. 21, 1943 [June 18, 1942], No. 15498/43. Class 40 (vi). [Also in Group XL (b)] An amplifier adapted to supply push - pull deflection poteritials to a cathode-ray tube preferably comprises valves 43, 63 with a common unbypassed cathode resistor 51 and direct couplings between the anodes and the deflecting plates of the cathode-ray tube 61. The amplifier is adapted to receive inputs from two different sources which each produce a balanced output. Switching means are provided alternatively to establish and to interrupt one of the inputs and interrupt and establish the other so that the joint output is balanced and alternately represents the inputs. Fig. 1 shows an arrangement in which the input A is applied to one valve 43 only of the pair, the other valve 63 being energized in opposite phase from the common cathode resistor 51. The second control voltage B is applied to the amplifier 63 so that the beam deflection of the cathode-ray tube is controlled by both this control voltage and also by the control voltage A applied to the amplifier 43. Direct coupling is preferably used, as shown, between the anodes of the valves 43, 63 and the deflecting plates of the cathode-ray tube. The voltages A, B impressed on the amplifiers 42, 63 are applied alternatively by the use of switch tubes 11, 74. To maintain the average output potential constant for both blocked and unblocked periods so that the grid bias of the amplifier 43, 63 remains constant, auxiliary valves 94, 107 may be provided and fed in appropriate phase with the switching potentials from the source 29. The switching potentials from the source 29 are applied in opposite phase to the grids of the switch tubes 11, 74 through the blocking condensers 36, 93. The grid circuits include series limiting resistances 27, 79 so that the tubes act as class A amplifiers and are conductive through opposite half-cycles independently of the amplitude of the switching voltage. In addition the grids 12, 86 are so biased so that the switch-tubes act as class A amplifiers for the inputs A, B but the bias may be varied to adjust the position of the image on the screen. The transformer 31 may be replaced by any other phase inverter and if the screen grids 17, 82 are not completely by-passed to ground they will assist the effect of the common biasing resistor 51 in maintaining the push-pull action of the valves 43, 63. Fig. 2 shows a modification in which the screen potentials are taken from the anode circuit of the other tube so that the bias of the valves 43, 63 is unaffected by the switching voltage. A switch 30 may be provided to enable the voltage A to be applied continuously to the cathode-ray tube 61 by an appropriate setting of the fixed bias applied to the grid resistors 28, 81. The switch tubes 11, 74 may use the anode output resistor as the source of screen grid potential for the other tube so that the switching of the tubes does not affect the grid bias supplied to the valves 43, 63. In a modification, Fig. 3 (not shown), series resistances are included in the screen grid circuits which are fed from the anode of the other tube of the pair so that if the anode current exceeds the screen grid current the bias of the grids 42, 66 remains at the same level during both halves of the switching cycle. In addition in this arrangement the instantaneous variations in screen grid current will assist the common bias resistor 51 in maintaining the push-pull action. Specification 684,950 is referred to.